Demand for higher functionality in today's consumer and business electronics continues to require greater complexity from modern circuits. The further requirement that such products be available at lower cost requires the reuse or multiple function use of as many components as possible. Many microprocessor based products such as telephones, digitally tuned radios, games and facsimile machines produce and/or transmit signals, the amplitudes of which must be regulated. Typically, this regulation process is performed under direct customer control, i.e., volume controls and the like, although there are implementations in which the regulation is done automatically. Today, there is an increasing trend toward push button control because of the low cost of physically providing a button and the circuitry to sense the state of the button. However, the digitally controlled regulators that are responsive to the state of the button continue to add substantial cost. Prior digital controls generally required multiple dedicated pins of a microprocessor or dedicated pins of a latched output port to set volume levels. These control systems typically utilized some type of digital to analog (D/A) converter to which a binary number is supplied as an input and from which a representative voltage magnitude of a predetermined set of voltage magnitudes is derived. This desired voltage magnitude is used to determine the gain of a voltage controlled amplifier.
Alternatively, multiple controls for shunting or series incorporating elements, typically resistors, are used to set the gain of an amplification stage. In order to reduce costs, component count and thus the number of gain settings was limited, resulting in noticeable discontinuity between magnitude levels. These prior gain regulation systems did, however, enjoy the advantage of simple and straight forward software development and implementation.
Another prior gain regulation system, disclosed in U.S. Pat. No. 4,868,519, uses microprocessor control to generate a variable duty cycle waveform that indirectly controls the drain to source resistance of an FET which is constantly monitored via a feedback loop, to regulate the gain of a two stage amplifier. Still other gain regulators, such as disclosed in U.S. Pat. No. 4,468,631, suggest the use of FETs or photo-resistors and photo-transistors for use in amplitude control apparatus. These systems are relatively expensive to implement because of their complexities. Additional cost is typically incurred by the introduction of nonessential elements that introduce indirectness of control and, most notably, in the use of feedback to monitor the actual value of the gain controlling factor or mechanism.